A balancer is a device used to counteract the force of gravity for hinged and/or articulated arm robots when such robots are likely to be activated manually or by some lower level power source. Such an occurrence typically takes place during teaching. Elimination or reduction of the effects of gravity allow the use of smaller power sources which reduces energy utilization and allows for better stability of servo-controlled mechanisms, such as the robot arm. With a balancer mechanism, articulated arm robots can be designed so that they can be manually led through their desired tasks without the use of a prime mover and the complexity associated with the control of the robot. As a result, the robot arm can be manually led through each desired task under low level power requirements.
The use of such balancers permits the use of smaller motors and gears since these elements no longer support the weight of the arm. This is desirable from a cost standpoint and allows for a more compact design which, in turn, allows greater accessibility to the workspace.
The prior art shows numerous gravity balancing mechanisms used on articulated arms and hinge mechanisms. One such arrangement utilizes counterweights for balancing the robot arm. However, the use of counterweights is oftentimes objectionable because of the added mass and resulting increase in arm inertia. For example, the inertia of a counterweight must be overcome every time the robot arm is to be moved in a different direction. Braking and change of direction of the robot arm is subject to inertial deceleration and acceleration forces due to the counterweights.
The following prior art patents disclose the use of counterweights as counterbalance mechanisms: the U.S. Pat. Nos. 2,344,108; to Roselund, 3,543,989; to Cooper and 4,402,646 to Le Rouzo.
Other prior art patents disclose the use of hydraulic and pneumatic balancers of both the active and passive type. Active balancers require an external power source to supply or absorb the balancing energy. Passive balancers store and release the balancing energy as required. Many of such hydraulic or pneumatic counterbalance mechanisms are relatively complex and costly. For example, the patent to Panissidi, U.S. Pat. No. 4,229,136 discloses an air pressure counterbalance system including an air-driven piston operated in the direction of the gravity axis as the manipulator hand is raised and lowered. The weights of different tools are programmed into computer memory and thereafter an air pressure regulator adjusts the counterbalancing force depending upon which tools are used by the manipulator.
Other U.S. patents which disclose hydraulic or pneumatic counterbalancing mechanisms include the U.S. Pat. Nos. 3,370,452 to Sack et al and 4,300,198 Davini.
When balancing is required within a small angle or within a single quadrant (i.e. from a horizontal to vertically upward orientation) a level of balancing can be obtained with a tension spring or passive pneumatic balancer. The following prior art patents disclose tension spring balancers which are useful within small angles of movement: the U.S. Pat. Nos. 3,391,804 to Flatan; 4,024,961 to Stolpe; 4,259,876 to Belyanin et al; 4,283,165 to Vertut; and 4,378,959 to Susnjara.
One objection to the use of conventional tension spring balancers is that they do not adequately balance the gravitational load. Also, it is inherent in most spring balancing methods that complete balance is possible only for one or two configurations of the arm and spring combination. As the robot arm moves away from that configuration in either of two possible directions, an unbalance is generated and progressively changes until the arm approaches a neutral orientation of zero gravitational moment.
Tension spring balancers frequently do not provide an acceptable level of balancing over extended angular movement of the robot arm. Because of this, oftentimes there are high actuation power requirements to overcome the effects of gravity on the robot arm. Such high actuation power requirements present a safety hazard if the mechanism should fall under the force of gravity when motor power is shut off. Consequently, such mechanisms are usually provided with brakes to alleviate the potential danger, or are overbalanced against gravity.
Spring stiffness, initial tensioning and anchor point location can be adjusted to give a higher degree of balance within a small angular displacement of the arm and also limit the maximum value of the unbalanced moment and/or its direction. Beyond that displacement the degree of unbalance grows relatively rapidly.
Despite the relative simplicity and relative inexpensiveness of conventional tension spring and passive pneumatic balancers, the balancers have oftentimes not been able to overcome their current angular limitations.
It has been found that high force compression springs operating on small moment arms can overcome this angular limitation problem and offer better balance over the entire range of travel of the robot's arm. Such a balancer utilizes a high force compression spring for balancing the arm of a robot. The compression spring is located inside of a balancer can which houses the spring. One end of the spring rests on the end of the can and the other end of the spring rests on a piston. The force of the spring is transmitted between the can and the piston. A sleeve bearing is located at the point where a piston rod connected to the piston passes through the can. The sleeve bearing prevents excessive friction and wear between the rod and the can as the rod moves relative to the can.
A cap is mounted on the opposite end of the can. In turn, the cap is attached to a block by means of a shaft and a journal bearing. The journal bearing allows the cap to rotate relative to the block. The block is attached to a wall member or casting by means of a threaded fastener. As the arm of the robot is moved the compression spring is loaded to balance the weight of the arm. Examples of such balancers are described in U.S. Pat. Nos. 4,592,697; 4,653,975; and 4,659,280 to Tuda et al assigned to the Assignee of the present invention.
The balancer described immediately above has the following limitations; (1) the journal bearing must be relatively large to accommodate the high spring force and to survive with a very limited angle of rotation which makes lubrication difficult; (2) the can of the balancer must be large enough to have clearance with the spring to prevent the spring from rubbing on it; (3) the balancer is heavy thereby making service and maintenance more difficult; and (4) the length and width of the balancer are relatively large for the limited space available inside a robot.